It's fun to grow your own onions from your own onion seeds. Somehow they just taste better when you do it yourself. In order to collect onion seeds, allow your onions to flower and allow the flowers to dry. Once the flowers have begun to dry out, collecting onion seeds is a simple matter. All you'll need are a pair of common garden shears and a plastic baggie.
Allow your onions to flower and wait until the flowers just begin to droop and wilt.
Use your garden shears to snip off the partially-dried onion flowers and place them in a large plastic baggie. Leave them in the baggie for 3 to 4 days until the flowers are completely dry.
Open the baggie and carefully shake the flowers while they are still inside the baggie. Seeds should fall from the flowers quite naturally at this point. If necessary, gently rub the flowers between your thumb and forefinger to release the seeds.
Remove the flower stalks and discard. Pour the contents of the baggie onto a sheet of newspaper and carefully separate the chaff from the seeds. Discard the chaff.
Put the seeds into a glass container with a tight-fitting lid. Seeds can be stored in a cool, dry location for up to a year.